Blown fuse indicators



A ril 14,1959 I E. SALZER E TAL 2,832,521

' BLOWN FUSE INDICATORS Filed Jan. 2, 1953 3 Sheets-Sheet 1 April 14, 1959 Filed Jan. 2. 1955 E. SALZER ET AL BLOWN FUSE INDICATORS 3 Sheets-Sheet 2 ,A ril 14, 19591 E. SALZER \ET AL 2,882,521

' BLOWN FUSE INDICATORS Filed Jan. 2. 1955 v s Sheets-Sheet 3 Fig.8

3 'INVENTORS.

United States Patent BLOWN FUSE INDICATORS Erwin Salzer, Brookline, and Paul C. Hitchcock, Groveland, Mass., assignors to The Chase-Shawmut Company, Newburyport, Mass.

Application January 2, 1953, Serial No. 329,428

12 Claims. (Cl. 340-250) This invention relates generally to electronic devices and more particularly to electronic devices for indicating whether or not an electric fuse has blown. The invention relates also to arrangements of the type comprising an indicator or pilot lamp for indicating the condition of an electric fuse.

The prior art electronic blown fuse indicators have serious limitations and can be applied only under special circumstances. It is, therefore, a general object of this invention to provide an electronic blown fuse indicator which is not subject to the limitations and drawbacks of the prior art blown fuse indicators and applicable to any kind of installation where it is desired to indicate the condition fuses are in.

A glow-lamp is an effective, rugged and inexpensive type of indicator lamp. As applied to indicating the condition of an electric fuse, glow-lamps have heretofore been subject to certain serious limitations. It is, therefore, a general object of this invention to provide a blown fuse indicator comprising an indicator glow-lamp which is not subject to the serious limitations of the prior art devices of this kind.

Where a glow-lamp is shunted across a fuse no potential is applied across the electrodes of the glow-lamp as long as the fuse is intact. Upon blowing of the fuse a potential is generally impressed upon the glow-lamp. This potential may be sufiiciently high to fire the glowlamp and may be maintained at a sufficiently high level to keep the glow-lamp lighted. The simple arrangement of a glow-lamp shunting a fuse is apparently inoperative as an indicator wherever the circuit voltage is less than the firing voltage of the glow-lamp. It is, therefore, another object of the invention to provide a glowlamp type blown fuse indicator which is operative irrespective of the magnitude of the voltage of the circuit into which the fuse is connected.

'The voltage of a circuit into which a fuSe is connected may be above the firing voltage of an indicator glowlamp shunted across the fuse and yet the glow-lamp will fail to operate as a blown fuse indicator upon blowing of the fuse if the recovery voltage across the terminals of the fuse upon blowing thereof is less than the firing voltage of the glow-lamp. Such a condition may occur in a number of instances and it is, therefore, another object of the invention to provide a glow-lamp type blown fuse indicator which is operative even if the recovery voltage prevailing across the terminals of the fuse is less than the firing voltage and less than the operating voltage of the glow-lamp.

Every interruption of a circuit by a fuse gives rise to a voltage transient. It is another object of the invention to provide a blown fuse indicator which is responsive to the voltage transients incident upon interruption of a circuit by a fuse.

The character of the voltage transients incident upon interruption of a circuit by fuses depends upon a number of factors and particularly upon whether the current under interruption is relatively low, as an overload of inadmissible duration, or relatively high, as a fault current in the nature of a short-circuit current. Where the current under interruption is relatively low arcing may last during a relatively long time, e.g. several cycles of a 60 cycles A.-C. current wave. If the current under interruption is relatively low and the circuit an A.-C. circuit, the arc voltage will always be less than the peak of the circuit voltage; the former will have distinct extinction peaks and reignition peaks and ripples between these two peaks which may be due to continual extinction and establishment of arc loops and of branches formed by the arc as it burns within the casing of the fuse and the pulverulent arc-quenching fillergenerally provided therein. On interruption of relatively high currents the arc tends to last but a fraction of a half cycle of an A.-C. current having a frequency of 60 cycles per second. Due to the rapid rate of change of current the inductive voltage tends to be high and this, in turn, is conducive to high are voltages. It is, therefore, another object of this invention to provide a blown fuse indicator which is responsive to the kind of voltage transient which occurs always incident upon interruption of an electric circuit by a fuse irrespective of whether the current under interruption is relatively low or relatively high.

Another object of the invention is to provide an electronic blown fuse indicator the operation of which is predicated upon the high frequency transient generated by the are formed in any fuse incident upon blowing thereof.

Still another object of the invention is to provide a blown fuse indicator of the glow-lamp type the glowlamp of which can be fired by a relatively small voltage transient incident upon blowing of a fuse.

A further object of the invention is to provide a blown fuse indicator of the glow-lamp type having a trigger transformer for firing the glow-lamp, the secondary winding of the trigger transformer being arranged in series with the discharge gap formed between the electrodes of the glow-lamp to minimize the voltage required for firing the glow-lamp.

A further object of the invention is to provide a blown fuse indicator including a glow-lamp circuit which is in effect both a relay circuit and an indicator circuit. 7

In certain electric systems, e.g. in electric systems for military aircraft, the distribution lines are formed by a plurality of conductors connected in parallel into the circuit. Each of these conductors is generally protected by a pair of fuses of which one is arranged at the supply end and the other at the load end of the distribution line. If one fuse arranged in such a multiple conductor blows the potential across the terminals of the fuse will remain substantially at zero since the fuse is shunted by one or more other conductors having a relatively low resistance. In such instances it has been impossible heretofore to apply glow-lamp type blown fuse indicators. It is, therefore, another object of this invention to provide a blown fuse indicator of the glow-lamp type'which lends itself for use in multiplex installations, i.e. installations comprising distribution lines which are made up of a plurality of conductors connected in parallel into an electric circuit.

While glow-lamps are satisfactory indicators in a number of instances yet, there are other instances where the Patented Apr. 14, 9

light emitted from a glow-lamp is not sufiiciently intense or where it is desirable to indicate by an electric signal other than a glow-lamp, e.g. by an incandescent lamp, that a fuse has blown. It is", therefore, another object of the invention to provide a blown fuse indicator having some of the advantages of glow-lamp indicators but including "an" electric signal other than a glow-lamp, e.g. an incandescent lamp.

1 Another object of the invention is to provide a blown fuselindicator wherein the indicator glow-lamp and all dria. number of elements of which the circuitry of the indicator consists is integrated into a self-sustained compact structural unit adapted to be readily installed in such locations as control panels, control boards, information centers, etc.

In the -so-called .gridFglow-lamps or grid-gl'ow-tubes control is'obtained by keeping the voltage between the grid'end the cathode below a' critical value which, if eirceedei'results in initiation of. the glow discharge within the'danip-or tube, or firing of the lamp ortube. The above critical voltage depends upon the anode voltage. When the above critical voltage is exceeded the glow starts between the cathode and the grid and then shifts to the anode. Grid-glow-discharge-tubes have the important advantage that they do not require anyfilament heating supply and they are widely accepted as relay tubes for low-current applications.

7 It is a further object of this invention to provide a relay *circuit having a glow-discharge diode rather than a glow-discharge triode or grid-glow-tube but having substantially the same operating characteristics as a relay circuit comprising a grid-glow-tube.

I Still another object of this invention is to provide a relay circuit comprising a tube of more simple construction, smaller bulk and greater ruggedness than any prior art relay grid-glow-tube but having substantially the same operating characteristics as 'a relay circuit including a grid-glow-tube.

Still another object of this invention is to provide a circuit including a gas-filled gas-discharge vessel having improved means for firing the gas-discharge vessel to initiate a gas-discharge across the spaced electrodes provided within' the vessel.

In circuits according to this invention blowing of a fuse is indicated by applying a D.-C. voltage to a gas 4 discharge lamp such as an indicator glow-lamp, of normally maintaining said voltage at a lower level than the voltage required to initially fire said lamp-but ata sufiiciently'high level to sustain a gas discharge through said lamp upon initial firing thereof, of resonating the high frequency transient incident upon blowing of a fuse and of applying said transient to said lamp to initially fire said lamp, to cause indication of blowing of said fuse. Preferably the high frequency voltage transient occurring acros'sa fuse incident upon blowing thereof is being picked-up and stepped-up, and the space between thespaced electrodes of a gas-discharge lamp is subjected to an'e'le'ctric field resulting from said stepped-up high frequency'voltage transient to initially fire said lamp, to cause'indication of blowing of said fuse. The indication produced by initial firing of the gas-discharge lamp by the-high frequency voltage transient incident upon blowing of the fuse is of a permanent nature since the gasdis'charge is maintained upon initial firing of the lamp by the D'.- C. voltage which is applied to it;

7 For the purpose of this invention use is preferably made of' gas-discharge lamps orglow-discharge lamps of the miniature or subminiature type. These lamps vhave a pair-of closely spaced internalelectrodes and are preferably filled' with neon or argon at pressures in the range from about 1.5 to 4 centimeters Hg. The power consumption of ,suchplamps is in the order of A to 3 .Tofenablethe lamps to be fired and to be epera'ted' at relatively lowvoltages the surfaces of the electrodes thereof may be treated with low-work-function materials. In such glow-lamps the glow-discharge is confined to the portions of the electrodes which have been treated in this manner. The currents which flow through a glow-lamp or glow-discharge lamp range from about 2 to 30 milliamperes, depending upon the type of lamp used. In some instances it is necessary to use glow-lamps in series with current-limiting resistors which may form an integral part of the lamp.

If a DC. voltage is applied to a gas filled diode or glow-lamp and the magnitude of the voltage progressively increased, the lamp is ignited or fired at a predetermined voltage V referred to as the firing voltage of the lamp. To extinguish the lamp the D.-C. voltage must be re duced below the firing voltage V The voltage V at which extinction of the glow-discharge takes place is referred to as the extinguishing voltage of the lamp. Hence e J The firing voltage V; of miniature glow-lamps suitable for carrying this invention into effect is about 75 volts and theextinguishing voltage V of such lamps is below 60 volts. A commercial subminiature glow-lamp having characteristic voltages of the above magnitude is the NE-Z glow-lamp manufactured by the General Electric Company. The operating voltage of'a glow-lamp is the voltage range at which a light-emitting glow-discharge occurs at the gap formed between the electrodes thereof, or to be more specific, at which light is emitted from the negative glow which covers the cathode of the glowlamp. The values of the firing voltage V and of the extinguishing voltage V are not absolutely fixed for a given glow-lamp but depend upon a number of factors as, for instance, the rate of change of voltage, or the previous history of the lamp, i.e. whether it was lighted or extinguished for some time before V; and V are being measured. While it requires a voltage of the above order above the extinguishing voltage to fire a gas-discharge lamp, the lamp can be fired by high frequency oscillations of smaller magnitude than the DC voltage that must be added to the extinguishing voltage to fire the lamp.

An ignited glow-lamp which seems to emit a steady light may, in fact, operate intermittedly in the fashion of a stroboscope at frequencies too high to be noticed by the human eye. The frequency of a glow-lamp which operates as a relaxation oscillator may be in the order of 15,000 cycles per second. It can be shown that the frequencyf of a glow-lamp operating as a relaxation oscillator is determined by the simplified equation.

g wherein e is the circuit voltage, 11- the resistance of the circuit in series with the glow-lamp, r the internal resistance of the glow-lamp (assumed to be a constaut); and C the capacitance:acrosstheglow-lamp. The first term in the aboveequation indicates the charging period of C and the secondterm the period of discharge of C. Further objects, advantages, features and characteristics of the invention will become apparent as the following description proceeds and the features of novelty which characterize the invention will be pointed out with pa:- ticularity in the annexed claims forming part of this specification.

For a better understanding of the inventionreference may be had to the accompanying drawingin which:

.Fig. 1 is a diagram of a blown fuse indicator circuit comprising an indicator glow-lamp having an external control electrode in addition to two internal electrodes Fig. 2 is a diagram of another blown fuse indicator circuit comprising an indicator glow-lamp having only twoeinternal electrodes, i.e. no. external control electrode; Fig. 3 is a diagram of a distribution oircuitflhavinga plurality of conductorsconnected in parallel into the circuit, each conductor being protected by one or more fuses and a common glow-lamp type indicator being provided for indicating whether any fuse located at the same end of a conductor in any of the plurality of conductors has blown;

Fig. 4 is a diagram of a polyphase circuit, each phase being protected by at least one fuse and each fuse being associated with a separate glow-lamp type blown fuse indicator to discriminate between blowing of fuses in different phases of the polyphase circuit;

Fig. 5 is a diagram of a circuit comprising two lines which are protected by fuses, the fuse in each line being associated with a gas-discharge lamp which is fired by the voltage transient incident upon blowing of the fuse, an electromagnetic relay which is energized upon firing of the gas-discharge lamp, and an incandescent lamp the circuit of which is closed by energization of the electromagnetic relay;

Fig. 6 is substantially a longitudinal section of a blown fuse indicator all essential parts of which are integrated into a self sustained structural unit;

Fig. 7 is substantially a longitudinal section of a blown fuse indicator wherein a glow-lamp and a step-up transformer are combined to form a self-sustained structural unit;

Figs. 8 and 9 are diagrams of the basic circuits according to this invention divorced from a fuse applicable where transients other than those incident upon blowing of a fuse are desired to control the flow of current in a circuit; and

Figs. 10 and 11 are diagrams of circuits for firing simultaneously a plurality of glow-lamps or glow-relays by a high frequency voltage transient.

Referring now to Fig. 1, reference numeral 1 has been applied to an electric conductor and reference numeral 2 to a fuse for protecting conductor 1. Fuse 2 is shunted by a shunt circuit 3. Shunt circuit 3 comprises two capacitors 4', 4" and the primary 5' of an air core step-up transformer 5. One end of the secondary 5" of transformer 5 is grounded while the other end thereof is connected to the external control electrode 6" of an indicator glow-lamp 6. Electrode 6" may be formed by a conductive coating on glow-lamp 6 or by a wire ring placed around the envelope of glow-lamp 6. The capacity of capacitors 4, 4" may be, for instance, in the order of .01 nfd. The secondary 5" of transformer 5 may be formed by a pancake type air core choke coil having an inductance in the order of 2.5 mh. The winding ratio between the primary 5' and the secondary 5" of transformer 5 may be in the order of 1:100. All the above figures are given as illustrations and not intended to limit the scope of the invention. Glow-lamp 6 comprises two internal electrodes 6' and 6" to which a constant D.-C. voltage is being applied continuously. The D.-C. voltage is less than the voltage required for initial firing of glowlamp 6, but is of a sufficiently high level for sustaining a glow discharge through glow-lamp 6 upon initial firing thereof. Transformer 5 is a firing or trigger transformer for firing glow-lamp 6. Once glow-lamp 6 has been fired by virtue of the high frequency voltage transient incident upon blowing of fuse 2 and by virtue of the operation of firing or trigger transformer 5, glow-lamp 6 remains lighted as long as the above D.-C. voltage is applied to the electrodes 6', 6" thereof. Firing or trigger transformer 5 causes firing of glow-lamp 6 irrespective of whether the current interrupted by fuse 2 is relatively low, or relatively high.

It will be noted that the circuit 2, 4', 5, 4" is normally closed, and opens only incident upon blowing of fuse 2, or removal thereof. As long as circuit 2, 4', 5', 4" is closed, no voltage transient in line 1 can induce a voltage in the secondary winding 5" of transformer 5, and thus glow-lamp 6 cannot be fired by any voltage transient occurring in line 1 except a voltage transient generated within the fusel2. Such a diiference in voltage can only be built-up after the link within fuse 2 has been destroyed by an excess current, i.e. an overload or short-circuit current, and been replaced by a gaseous gap. In other words, voltage transients initiated at some other points of the system, either by switching operations or faults, have no effect whatsoever upon indicator glow-lamp 6, the operation of the latter being predicated upon a destruction of the fuse link or removal of the fuse.

In Fig. 1 the D.-C. circuit for operating glow-lamp 6 has been indicated merely by a pair of and signs. Either a battery or a rectifier unit may be used for supplying the D.-C. voltage required for operating glow-lamp 6. In the embodiment of the invention shown in Fig. 3 glow-lamp 6 is shown as being supplied by a half wave rectifier unit.

Firing of glow-lamp 6 is apparently predicated upon an increase of the intensity of the electric field normally prevailing across the gap formed between the electrodes 6, 6" to the point where the dielectric strength of the gap breaks down, thus giving rise to a gaseous glow-discharge within lamp 6. If the voltage applied to the external electrode 6" of glow-lamp 6 were of a static nature, an excessive voltage would be required to effect a breakdown of the gaseous interelectrode gap; but since the voltage transients incident upon blowing of fuse 2 have a high frequency, the voltage required for breaking down the gaseous interelectrode gap in glow-lamp 6 is relatively small. The arc formed in fuse 2 during the process of interruption by the fuse 2 is in effect a highfrequency generator and the parameters of the circuit 4', 5', 4-" may be chosen in such a way that the circuit is roughly in tune with the frequency of the oscillations produced by the interrupting arc in fuse 2. Transformer 5- operates as a Tesla type high-frequency transformer to step-up the voltage of the arc-generated high frequency oscillations. Not only the arc voltage but also the transient recovery voltage which prevails across the terminals of fuse 2 immediately upon ultimate current zero may have harmonics which may have a relatively high fre quency and may be capable of breaking down the dielectric strength of the interelectrode gap or space formed between electrodes 6' and 6". Because use is made for firing glow-lamp 6 of oscillations having a relatively high frequency, the windings of transformer 5 should have a relatively high lumped self-inductance but relatively little distributed capacitance. In order to meet with these requirements it is desirable to use so-called honey comb coils for the windings 5 and 5" of transformer 5.

The DC. circuit of glow-lamp 6 includes a push-button switch 19 normally biased to the closed position thereof. Glow-lamp 6 is extinguished upon firing thereof by opening of push-button switch 19. Thus push-button switch 19 is a re-set switch and must be opened before a blown fuse is replaced by a new sound fuse.

The same reference signs have been applied in all figures to designate like parts or elements, and therefore Fig. 2 and the following figures need to be described in detail only to the extent that they differ from Fig. l.

Numeral 7 in Fig. 2 has been applied to a voltage divider or potentiometer. The full voltage V across potentiometer 7 is larger than the firing voltage V; of the glow-lamp 6, i.e.

The voltage V across the right hand terminal of the potentiometer and an intermediate potentiometer tap is smaller than the firing voltage V, but larger than the extinguishing voltage V i.e.

Fuse 2 in conductor 1 is shunted by the primary winding 5 of trigger transformer 5 and the two series capacitors 4' and 4". Trigger transformer 5 is a step-up transformer and one terminal of the secondary winding 5" thereof is connected to the electrode 6' of glow-lamp 6, whereas the other terminal of the secondary winding 5" is con!- nested "rorhemovable Contact of a change-over switch 8. Lead '9 connects one terminal of po'tentiometer 7 to electrode .6" of:glow1amp .6. In the po'sition of change-over switch 8 shown in the drawing the voltage V is applied across glow-lamp 6. When change-over switch 8 is turned in clockwise direction the voltage V is applied across glow-lamp -.6. Normally change-over switch 8 remainsinthe positionshown and is only moved in clockwise direction when it is desired to test whether the circuit .of the glow-lamp 6,.isintact and operative.

Trigger transformer has been shown as being proyided with a core 5" comprising a ferrous substance. This enables to miniaturiz'e trigger-transformer 5 and to save copper on the windings 5, 5" thereof. Core 5" may consist .of .a .plastic substance used as a binder to eeme'ntfine ferrous particles into a solid core structure. Thecore' material known'by its trade name Ferrite has pioved to be sa-isrscmr for making up trigger-transformer 5.

It will be noted that the third or external electrode 6 present in the circuit shown in Fig. .1 has been eliminated in the circuit shown in Fig. 2. In the arrangement shown in Fig. 2 the transient voltage induced in the secondary winding .5 of trigger transformer 5 is superimposed upon the D.- C. voltage V of potentiometer 7 and causes a dielectric breakdown of the gap formed between the electrodes6' and 6" of glow lamp 6. The transient A.-C. voltage across the secondary winding 5" of transformer 5 may be less in the arrangement shown in Fig. 2 than in the arrangement shown in Fig. 1 to cause firing of the respective glow-lamp 6. The circuit of glow-lamp 6 includes push-button switch 19 normally biased to the closed position thereof and manually operable to break the circuit.

vIt will be noted from Fig. 2 that glow-lamp 6 is provided with an electrostatic shield 51- to exclude the effect of extraneous electromagnetic fields from the space encompassed by the glow-lamp. Shield 51 is connected to the negative pole of potentiometer 7. Provision of electrostatic shielding of the glow-lamp is generally indicated inany application. It becomes imperative where the D.-C. voltage across the glow-lamp is close to the firing voltage thereof for, under such conditions, no effects of extraneous electromagnetic fields upon glow-lamp 6 can be tolerated.

Fig. 3 refers to an electric system comprising a distribution line-including three conductors 1, 1", 1' connected in parallel into an electric circuit. Such multiple distribution lines are used to advantage in certain applications as, for instance, military aircraft. A group of three'fuses 2", 2, 2" i's'ins'erted into conductors 1, 1", 1'" at one of the .ends-the'supply end-thereof, and another group of three fuses 2', 2", 2" are inserted into conductors-1", 1", .1' at the other of the endsthe load ends-wthereof. .A common-indicator glow-lamp 6 is pro Vided: to Show whether one or more of the fuses of the upper or supply" endgro'up of fuses 2, 2", -2 has blow n.- A similar indicator glow-lamp with its requisite auxiliaryiparts may be'provided to show whether one or moreof thepfuses:of 'the' lower :or load end group of fuses 2', 2", 2" has blown, but this last mentioned glowlamp and its auxiliary parts'have been omitted from Fig. 4. Each of the fuses 2', 2", 2 is shunted by a shiintll', 3", 3". Inductance 5 is common to shunts 3', 3"; 3'". Shunts 3', 3", 3' comprise the joint capacit'oi- 4'a'n'd'the separate capacitors 4, 4", 4" for insulating inductance 5 from conductors 1', 1", 1'. Inductance 5 forms the primary'winding of a step-up firing transformer 5 comprising the secondary winding 5" and core 5. One terminal of-the secondary winding 5" is connected to electrodefi" ofglow-lamp 6. The electrode 6-" of glow lamp -6 is grounded. Glow-lamp 6 is notmally'supplied with a D;-C. voltage lessthan the firing voltage Yfbut in excess of the" extinguishing voltage V eraser by means of are'c'tifier unit generally'designated by the reference numeral 10. The two terminals 11 of rectifier unit 10 are connected to an A.-C. current-source. The rectifier unit 10 comprises the rectifier 12 proper which may be of any known type as, for .instance, a group of selenium or copper oxide rectifier cells or'agaseous discharge rectifier tube. The rectifier unit 10 comprises further a glow-discharge voltage stabilizer lamp 13, two capacitors 14 and 15 and one resistor 16. One D.-C. terminal of the rectifier unit 10 is connected to the inductance or secondary transformer winding 5"; one D.-C. terminal of rectifier unit 10 is grounded. The voltage across the two D.-C. terminals of rectifier unit 10 is less than the firing voltage V but in excess of the extinguishing voltage V of glow-lamp 6.

Any incfease in the D.-C. voltage on the unregulated side of the rectifier tends to .result in an increase of the current through tube 13 and an increase of the voltage drop across resistor 16. The latter tends to limit the voltage across tube 13. A small increase in voltage on the unregulated side of the rectifier results in a'large increase of the current through tube 13 and consequently in a large increase of the voltage drop across resistor 16. Therefore, small variations of the supply voltage cause compensating variations of the current through tube'13 and the voltage on the regulated side of the rectifier, i.e. the voltage across the terminals of tube 13 remains subs'tantially' constant. A tube which may be used for so stabilizing the voltage applied to glow-lamp 6 is the OA3/VR75.

The closer the voltage of the D.-C. supply applied to the glow-lamp 6 to the firing voltage of the glow-lamp the more important it is to achieve a close regulation and stabilization of the D.-C. votlage supply.

Upon blowing of either of the fuses 2, 2", 2" the resulting voltage transient energizes inductance 5' which in turn energizes inductance 5". The D.-C. voltage produced by the rectifier unit 10 is increased by the high frequency transient voltage induced in inductance 5" and the resulting voltage breaks down the gaseous gap formed between electrodes 6 and 6" of glow-lamp 6. The relatively high transient A.-C. voltage generated in inductance or transformer winding 5", while effective in firing glow-lamp 6, does not tend to produce a large current flow through the glow-lamp circuit because of the latters high impedance. While the impedance of the D.-C. circuit of glow-lamp 6 is relatively high, the ohmic resistance of the circuit is relatively small, resulting in a relatively small voltage drop in that circuit and a relatively intense light emission from the glow-lamp.

Upon replacement of the blown fuse by a new one push-button switch 19 is temporarily pushed open to open the circuit of glow-lamp 6 to cause extinction thereof. When push-button switch 19 is released, the circuit of glow-lamp 6 is automatically reclosed by the action of a biasing spring (not shown).

The polyphasc circuit shown in Fig. 4 includes three conductors 1', 1", 1" each provided with a fuse 2,-2, 2". Fuses 2', 2", 2" are shunted by shunts 3, 3", 3". Each of these shunts includes two capacitors 4', 4", and an inductance 5. Each inductance 5 forms the primary winding of a transformer 5 having a secondary winding 5". An indicator glow-lamp 6 is associated with each of the fuses 2', 2", 2" to indicate the condition thereof. One end of each secondary winding 5 of each transformer 5 is connected to the anode 6' of a glow lamp 6, the cathode 6" of which is grounded. The opposite end of each secondary winding 5" of each transformer 5 is connected to the common lead 40 including manually operable push-button switch 19 and connected to the plus pole of a D.-C. source of appropriate voltage. The negative pole of said D.-C. source is grounded at 41.

The glow lamp 6 at the left indicates whether or not fuse 2 has blown, the glow-lamp 6 at the center indicates whether or not fuse 2" has blown and the glow-lamp 6 at the right indicates whether or not fuse' 2 has blown.

Referring now to Fig. 5, fuse 2' is inserted. into line 1 and fuse 2" into line 1". Fuse 2 is shunted by shunt 3' and fuse 2" is shunted by shunt 3". Shunts 3' and 3" each comprise a pair of capacitors 4' and 4" and an inductance 5'. Inductances 5 each form the primary winding of an air core transformer 5 having a secondary winding 5". Windings 5" are connected to the anodes 6 of glow-lamps 6. The cathodes of glow-lamps 6 are connected to sensitive relays, e. g. telephone type relays 21 and 22. Relays 21 and 22 are grounded at 42 and 43, respectively. Relay 21 comprises an armature 45 controlling an electric circuit including the incandescent lamp 24 and the conductor 26. Relay 22 comprises an armature 46 controlling an electric circuit including the incandescent lamp 25 and the conductor 27.

Upon blowing of fuse 2' the left transformer 5 is energized by the blowing transient of the fuse. This causes firing of the left glow or gas-discharge lamp 6 and energization of relay 21. As a result thereof the circuit of lamp 24 is closed and lamp 24 thus indicates that fuse 2 has blown. The right transformer 5, the right glow or gas-discharge lamp 6, relay 22 and lamp 25 coact in like fashion to indicate whether or not fuse 2" has blown.

Incandescent lamps 24 and 26 may be arranged at a point of the system remote from fuses 2' and 2" and relays 21 and 22. It is also possible to substitute gasfilled triodes for the diodes shown in Fig. 6. Such a triode would be connected in the same way as shown in Fig. 1 where the control electrode is arranged outside rather than inside the gas-discharge lamp. If desired any other kind of electrically operated indicator or annunciator may be substituted for lamps 24 and 25 as, for instance, an acoustic signalling device, or a so-called magneto-drop as used in manually operated telephone switchboards.

The structure shown in Fig. 6 comprises a glow-lamp 6, a transformer 5 and a pair of capacitors 4' and 4" which are connected in the same fashion as like elements bearing the same reference numerals shown in Figs. 2, 4 or 5. The core 5" of transformer 5 and the windings thereof are arranged coaxially with respect to glow-lamp 6. Capacitors 4' and 4" are of lenticular shape, known in the trade as Hi-Kaps, manufactured by the Centralab Company, and the glow-lamp 6 is the General Electric Company NE2 lamp. Fig. 6 has been drawn on a scale approximately 2:1. Parts 6, 5, 4' and 4" are enclosed in a tubular shell or housing 30 of insulating material, i.e. vulcanized fiber, having on the left side thereof a metallic sleeve 32 and being closed on the right side thereof by a circular plate or disc 47. Reference numeral 31 has been applied to a steel panel having a punched out hole 31' through which sleeve 32 projects. The outside of sleeve 32 is screw threaded as indicated at 33. Panel 31 is clamped between a shoulder 48 formed by tubular housing 30 and a screw-nut 34 and a washer 35, respectively, both mounted on sleeve 32. Glow-lamp 6 projects from housing 30 into sleeve 32 and slightly beyond sleeve 32. The purplish glow of lamp 6 can readily be ascertained by viewing sleeve 32 from the left end thereof. Lead 53 of glow-lamp 6 is connected to sleeve 32 which forms an effective electrostatic shield for lamp 6. Lead 54 of transformer 5 is connected to a rivet 59 projecting transversally across housing 30. The

capacitors 4' and- 4" have leads connected in a similar fashion to rivets projecting transversally across housing 30. Reference numeral 58 has been applied to one of these rivets. The other rivet is angularly displaced and therefore cannot be seen. I

The structure shown in Fig. 7 comprises a glow-lamp 6 having a pair of internal electrodes 6' and 6" and a transformer 5 having a ferrous core 5" both enclosed in a common casting 36 of a suitable transparent plastic. Shunt leads 3 extending from the rear end of casting 36 include a pair ofcapacitors 4', 4" of cylindrical shape. The outside of casting 36 may be screw-threaded, if desired, to make it possible to mount it on panelboards in a way similar to the way in which the structure of Fig. 7 is mounted. Where there is some space between the point where the fuse is located and the point where the glow-lamp is located, it may be desirable to arrange the capacitors 4' and 4" relatively close to the point where the fuse is located. The arrangement of Fig. 7 is de signed to comply with this requirement.

The blown fuse indicators according to this invention can be applied to fuses which are arranged in an A.-C.

circuit as well as to fuses which are arranged in a D.-C. circuit. In D.-C. circuits having but a small circuit voltage the voltage transients produced on blowing of a fuse are generally suflicient to fire the glow-lamp, or glowrelay. Tests made in aircraft type D.-C. distribution system having a circuit voltage of 30 volts proved consistent performance of the glow-lamp indicator irrespective of whether the current interrupted by the fuse was an over load, or a fault current of short-circuit current proportions. Wherever it is'intended to indicate blowing of a fuse arranged in a circuit having a circuit voltage of less than volts, e.g. in an aircraft-type D.-C. distribution system having a circuit voltage of 30 volts, glow discharge-tubes or diodes should preferably be used having electrodes prepared with low-work-function material to reduce the firing voltage of the tube to the range of 75 volts and the burning voltage thereof to the range of 60 volts. Such glow-discharge tubes can readily be fired by a small additional voltage of relatively high frequency which may be in the radio frequency range.

Referring now to Figs. 8 and 9, numeral 60 indicates generally a transformer having a primary winding 61 and a secondary winding 62. Capacitor 4 is connected in series with the primary winding 61 of transformer 60. To limit the overall distributed capacity of the secondary Winding 62 the latter is made up by a number of sections which are serially connected into the secondary circuit. Each section consists of a plurality of layers and each section is relatively deep and narrow. Winding 62 has a relatively high impedance to currents within a certain frequency range but a relatively low ohmic resistance and operates in the fashion of a high-frequency choke coil.

Fig. 9 differs from Fig. 8 merely by the addition of an electromagnetic relay generally indicated by the reference numeral 21, having an armature 45. One terminal of the core energizing winding of relay 21 is connected to the cathode of the glow-discharge tube 6 while the other terminal of the core energizing winding of relay 21 is grounded at 42.

It may be desirable to operate two or more glow-discharge tubes in parallel, either for the purpose of increasing the light emission, or for the purpose of increasing the current output to be able to energize by the sum total of the currents of a plurality of tubes a none too sensitive electromagnetic relay.

' Where the voltage applied to one or more substantially identical glow-discharge tubes is in excess of the firing voltage thereof, no diificulties are encountered in operating a plurality of glow-discharge tubes connected in parallel into a circuit. However, where a plurality of substantially identical glow-discharge tubes are connected in parallel into a D.-C. circuit applying a smaller voltage to the glow-discharge tubes than the firing voltage thereof and where the plurality of glow-discharge tubes is intended to be fired by a pulse or transient derived from another circuit, difficulties may be encountered in firing all of the tubes. These difficulties may be attributed to the fact that the firing voltages of substantially identical glow-discharge tubes are not identical on account of unavoidable manufacturing tolerances.

' The glow-discharge tube having the smallest firing volt- 11 i the extinguishing voltage thereof, no glow-discharge t fibein additionto the first-firedtube will ever .be fired by the pulse or transient which fired thefirst-firedglowdischarge tube. a v H i The circuits-shown in Figs. 10 and 11 enable to fire and to operate simultaneously .-a plurality of substantially identical glow-discharge tubes 6 each having a. pair of internal electrodes 6 and 6" and an external shield 51 and each having a slightly different firing voltage.

Referring now to Fig. 10 fuse 2 in line 1 is shunted by capacitor 4', inductance 5' and capacitor 4". Inductance 5 forms the primary winding of a trigger transformer for glow-discharge tube 6 including the two secondary windings 5". The point between the secondary windings 5 is grounded and the terminals of-the'se'condary windings 5i which are above ground potential are each connected to one of the electrodes 6' of the two tubes 6. vThe electrodes 6" of the two tubes 6 are connected to the positive pole of a D.-C. supply whereas the electrodes 6" of the two tubes 6 are connected to the grounded negative pole of the vDEC. supply. The voltage of the l i-C. supply is less than the firing voltage of the two tubes 6 but lies within their range of operatis o a e -i i, i V

H Assuming now for the purpose of illustration that due tounavoidable manufacturing tolerances the firing voltage oifthe upper tube 6 is slightlyless than thefiring voltage of the lower tube 6. The voltages in both secondary windings 5" of the trigger transformer being identical, the upper tube 6 will fire first. The secondary windings 5" operate in the fashion of high-frequency choke coils, i.e. they preclude the flow of a high freg uetreyv current-across theupper tube 6 and consequently a break-clown of the voltage across the electrodes of the upper tube 6 to a relatively low value. Since firing of the first-fired glow+dischargetube does not result in an immediate reduction of the voltage across the electrodes thereof and across the electrodes of the other glow-discharge tube, the latter will be firedby the voltage transient incident upon blowing of fuse 2 immediately after firing of the first-fired glow-discharge tube. V

The circuit shown in Fig. 11 operates in a similar i h pi a t e siren. 9 i 9- h fgmier cirsuit comprises an electric distribution line 1 into which fuse 2 is inserted. Fuse 2 is shunted by capacitor 4', the primary winding 5' of a trigger transformer and capacior "Q The secondary winding 5" of the trigger transformer is connected in series with the gas-discharge gap a med b ween e wc e e r de f h .u pe most glow-discharge lubed. Each of the two additional or lower glow-discharge tubes 6 is associated with a h f e l oan d series withthesas-d sch rse gap forrned be t'ween the two electrodes 6" 6f thereof. The Secondary winding 5" of the trigger trangformeris inthe nature of a choke coil and itis'connected inparall el with qthe two choke coils 5 associated with the two lower glow -discharge tubes 6. Upon -firing of any of he hf s ewi r tubes 6 ea of t m 29f t e twooth'ers assign frequency componentofthefiring remains effective since each of the windings 5 operates as a choke coil within a predetermined frequality range an precludes any appreciable high ie,- quenc'y discharge through any of the glow-discharge It willfbe understood that we have illustrated and ,de sciib ed hereinipreferred embodiments only of pur inyentio n, and that various alterations may' be rnade .iu the detailsthereof.withoutdeparting from the spiritand scopejof our invention asdefined in theappendedclaims: WeflaiW t i 1. In combination, an electric distribution circuit; a 'i i f t i i i i e i a r iis i idis ibsuiqn it cjom'p'rising .rneans for initiating ;an t arc therein Patato'r-yf Ito; interrupting said distribution; circuit an mammary-arena "responsive-"ta "the range of eequeai's sass-sat generated by said arc in 'said interrupting device, said oscillatorycircuit including said interrupting device, a pair of capacitors each connected with one terminal thereof to one .of the terminals of said interrupting device, .,'and an inductance interconnecting the other terminal of each of said pair of capacitors; a gas discharge vessel; a D. -C. source normally applying a voltage to saidyessel'less than the firing voltage but in excess of the extinguishing voltage thereof; and trigger means under the control of said oscillatory circuit for triggering said gas discharge vessel in response to arc initiation in said interrupting device.

2. In combination, a source of high-frequency oscillations'; an oscillatory circuit substantially in resonance with the frequency of said source energized by said source; a glow dischargediode having a pair of electrodes prepared with a low-work-function material to reduce the firing voltage thereof to the range of 75 volts and the burning voltage thereof to the range of 60 volts; a D.-C.

circuit normally applying a voltage across said diode less than said firing voltage but in the order of said burning voltage; an inductance serially arranged with said diode in said D. C. circuit; and coupling means for encrgizing said inductance from said oscillatory circuit to firesaid diode.

a 3 In combination, an electric circuit; a fuse arranged in said circuit for overcurrent protection thereof; an oscillatory circuit tuned to resonate in the high frequency range, said oscillatory circuit including said fuse, a pair of capacitors each connected with one terminalthereof to one of the terminals of said fuse, and an inductance interconnecting the other terminal of each of said pair of capacitors; a glow discharge tube; a D.-C. sourcenormally applying a voltage to said tube less than the firing voltage but in excess of the extinguishing voltage thereof; and trigger meansincluding a step-up transformer under the control of said oscillatory circuit for triggering said glow discharge tube in response to are initiation in said fuse.

4. In combination, an electric distribution circuit; a fuse arranged in said distribution circuit for overcurrent protection thereof; an oscillatory circuit tuned to resonate in the high frequency range of the interrupting arc formed inside said fuse incident to blowing thereof, said oscillatory circuit including said fuse; a pair of capacitors each connected with one terminal thereof to one of the terminals of said fuse, .and a first inductance interconnecting the other terminal of each of said pair of capacitors; a gas-discharge tube; a D -C. source normally applying a voltage to' said tube less than the firing voltage but in excess of the extinguishing voltage thereof; a second inductance magnetically coupled with said first inductance to jointly form a step-up transformer, said second inductance being serially connected with said gas-discharge tube in the circuit of said D.-C. source; a normally closed manually operable switch; and an electromagnetic relay having an energizing winding; said switch and said en'en gizingwinding being also serially connected with said gas discharge tube in the circuit of said D.-C. source.

5. In combination, an electric distribution circuit; a fuse connected into said circuit for overcurrent protection thereof; an oscillatory circuit tuned to resonate in the high frequency range, said oscillatory circuit including said fuse -a pair of capacitors each connected with one terminal thereof to one of the terminals of said fuse, art-inductance interconnecting the other terminal of each of said pair of capacitors; a plurality of substantially identical :glow. discharge diodes; -a D.-C. circuit normally a'pplyinga yoltage to each of said plurality ofdiodcs less than the firing voltage thereof but in excess of the extin guishing voltage thereof; a plurality of high impedance windings each arranged in said D.-C. circuit in series with on et said plurality of diodes to limit thefiow of high frequency currents through each of said plurality of diodes; and means for stepping-up voltages in said oscillatory circuit and applying the stepped-up voltages across each of said plurality of diodes to substantially simultaneously fire said plurality of diodes in response to blowing of said fuse.

6. In combination, an electric circuit; a fuse arranged in said circuit for overcurrent protection thereof; an as cillatory circuit tuned to resonate in the high frequency range, said oscillatory circuit including said fuse, a pair of capacitors each connected with one terminal thereof to one of the terminals of said fuse and means for conductively interconnecting the other terminal of each of said pair of capacitors, said means including the primary Winding of a step-up transformer; a glow discharge diode; a D.-C. source normally applying a voltage to said diode less than the firing voltage but in excess of the extinguishing voltage thereof, and said step-up transformer including a secondary winding serially connected with said glow discharge diode in the circuit of said D.-C. source to cause firing of said diode by high frequency oscillations generated in said fuse incident to blowing thereof.

7. In combination, an electric circuit, a fuse arranged in said circuit for overcurrent protection thereof; a shunt arranged across said fuse, said shunt including a first inductance and a pair of capacitors insulating said first inductance from said conductor, said fuse, said first inductance and said pair of capacitors forming an oscillatory circuit tuned to resonate in the high frequency range; a glow discharge diode; a D.-C. source normally applying a voltage to said diode less than the firing voltage but in excess of the extinguishing voltage thereof; and a second inductance magnetically coupled with said first inductance to jointly form a step-up transformer, said second inductance being serially connected with said glow discharge diode in the circuit of said D.-C. source.

8. In combination, an electric distribution circuit; a fuse arranged in said distribution circuit for overcurrent protection thereof; an oscillatory circuit tuned to resonate in the high frequency range of the interrupting arc formed in said fuse incident to blowing thereof, said oscillatory circuit including said fuse, a pair of capacitors each connected with one terminal thereof to one of the terminals of said fuse and means for conductively interconnecting the other terminal of each of said pair of capacitors, said means including the primary winding of a high-frequency step-up transformer; a glow discharge diode having a pair of electrodes prepared with a low-work-function material to reduce the firing voltage thereof to the range of 75 Volts and the burning voltage thereof to the range of 60 volts; a D.-C. source normally applying a voltage across said diode less than said firing voltage but in the order of said burning voltage; and said step-up transformer including a secondary Winding serially connected with said glow discharge diode in the circuit of said D.-C. source.

9. In combination, an electric distribution circuit; a plurality of conductors arranged in parallel in said circuit; a plurality of fuses each inserted into one of said plurality of conductors for overcurrent protection thereof; a plurality of shunts one across each of said plurality of fuses; an inductance common to said plurality of shunts; block capacitors arranged in each of said plurality of shunts for insulating said first inductance from each of said plurality of conductors, said plurality of fuses, said inductance and said capacitors forming a plurality of oscillatory circuits tuned to resonate in the high frequency range; a common glow discharge tube for indicating visually blowing of each of said plurality of fuses; a D.-C. source normally applying a voltage to said tube less than the firing voltage but in excess of the extinguishing voltage thereof; and trigger means including a high-frequency step-up transformer under the control of each of said plurality of oscillatory circuits for triggering said glow discharge tube in response to blowing of each of said plurality of fuses.

10. In combination, an electric distribution circuit; a plurality of conductors arranged in parallel in said circuit; a plurality of fuses each inserted into one of said plurality of conductors for overcurrent protection thereof; a plurality of shunts one across each of said plurality of fuses; a first inductance common to said plurality of shunts; capacitors arranged in each of said plurality of shunts for insulating said first inductance from each of said plurality of conductors; said plurality of fuses, said first inductance and said capacitors forming a plurality of oscillatory circuits tuned to resonate in the high frequency range; a common glow discharge tube for indicating visually blowing of each of said plurality of fuses; a D.-C. source normally applying a voltage to said tube less than the firing voltage but in excess of the extinguishing voltage thereof; and a second inductance magnetically coupled with said first inductance and serially connected with said glow discharge tube into the circuit of said D.-C. source.

11. In combination, a panel board, a perforation in said panel board, a tubular member projecting through said perforation and clamped to said panel board, a glow discharge diode arranged inside of said member adjacent the end thereof clamped to said panel board, a highrequency step-up transformer also arranged inside of and supported by said member, means for serially connecting said diode and the secondary winding of said transformer, and a pair of block-capacitors connected to the primary winding of said transformer enabling to energize said primary winding from a high-frequency source.

12. In combination, a tubular member of insulating material comprising a shoulder portion, a glow discharge diode arranged inside of said member adjacent one end thereof, a pair of capacitors arranged inside said member adjacent the other end thereof; a high-frequency step-up transformer arranged inside of said member in the space situated between said glow discharge diode and said pair of capacitors, said transformer comprising a primary winding connected with each terminal thereof to one of said pair of capacitors and a secondary winding connected to one of the electrodes of said diode, and annular clamping means at said end of said member where said diode is located adapted to clamp said shoulder portion of said member against an annular supporting surface.

References Cited in the file of this patent UNITED STATES PATENTS 1,138,170 Allgood May 4, 1915 2,001,432 Olving May 14, 1935 2,317,030 Colvin Apr. 20, 1943 2,447,658 Marbury et a1 Aug. 24, 1948 2,478,908 Edgerton Aug. 16, 1949 2,565,271 Sealander Aug. 21, 1951 FOREIGN PATENTS 886,501 France July 4, 1943 

